I've heard this question many a time, but never seen a definative answer. Are these semantics? Marketing air? What is the difference? Dimensions?
3 years ago I took the Disney Magic from Florida to the Bahamas and back. Smooth as silk. I would have been happy to take it to Southampton! But it's not an "ocean liner". The QM2 looks similar in many ways to Disney's Boat, except TWICE as big?
I don't think you'd be too happy on the Disney Magic in Winter on the North Atlantic, for which the liners were designed. I'd suggest "The Only Way To Cross", by John Maxtone-Graham as a good read... the definitive book on Ocean Liners.
The answer to your question is subject to many opinions, and is the source of endless debate on the boards... but in essence it is the vessel's draft (about 35 feet or so on the QE2), its cruising and maximum speeds (considerably higher on a liner than on a purpose-built cruise vessel) and the design of the liners' hulls, all configured to give efficient, speedy and stable performance over long periods of uninterrupted running in changeable waters.
You'll find that the Disney Magic "boat" and most other "cruise" ships have much slower speeds, shallower drafts and flatter bottoms, making them ideal for the purposes for which they were built but not the first choice on the stormy North Atlantic.
As for the difference between a "ship" and a "boat", alluded to in the previous sentence, now there's another subject for debate. I'm reminded of a U.S. Supreme Court Justice's comment on pornography... "I cannot define it, but I know it when I see it". I guess I feel the same about the difference between a boat and a ship. I can't define the difference, but I know a ship when I see one.
In the US the difference between a ship and a boat is based on tonage. I do not remember the exact figure for the changeover.
In the UK it is a little bit more complex but a captan friend has the saying "You get into a boat and on to a ship".
As for Liner vs Cruise Ship..
In a classic case of form follows function, liners are designed to cross oceans in all weathers while cruise ships are designed to potter round small areas of sea. Liners have finer lines for both speed and shrugging off weather. A lot more power for speed. A liner will often only have portholes within fifty feet of the waterline for safety in heavy weather. Liners are also a lot stronger than a cruise ship of the same size.
A liner will often be deeper in the water because they are heavier for the same ship size because of the stronger construction, larger engines and larger fuel load. The narrow, pointy hull shape also means that to get the same dispacement you need a deeper draught. This is an issue which actualy helps seakeeping but does not help the fuel bills.
As my grandfather was on tthe design team for the QE2, the difference between Cruise Ships and Modern Ocean Liners is that they are built with reinforced steel to withstand the pounding of the open ocean. That's all.